1. Field of the Invention
The present invention relates to the analysis of an underground earth formation and, more particularly, to receiving signals from the formation to estimate the resistivity of the formation.
2. Description of the Related Art
Exploration and production of hydrocarbons requires accurate and reliable measurements of a geologic formation, which may contain a reservoir of the hydrocarbons. In general, a borehole is drilled into the earth to gain access to the formation to perform the measurements. Measurements performed in the borehole are generally referred to as well logging.
In one embodiment of well logging referred to as logging-while-drilling or measurement-while-drilling, a logging instrument or tool is disposed at a bottom hole assembly at the distal end of a drill string. As the drill string rotates to drill the borehole, the logging tool performs the measurements. The measurements are generally logged with a depth at which they were performed. Together, the measurements and associated depths create a well log. The well log may be presented graphically or as an image.
One type of downhole measurement is of electrical properties of the earth formation. The electrical properties provide information about the geologic materials that make up the formations, and about their likely oil, gas and water content.
The electrical properties of the earth formation may be measured by an induction well logging technique. In induction well logging, a transmitter emits electromagnetic energy into the formation. The changing magnetic fields associated with the energy induces alternating circulating currents in the formation. The alternating circulating currents in turn emit electromagnetic energy that is received by a receiver via an antenna. Characteristics of the received electromagnetic energy are then related to the electrical properties of the formation.
In conventional well logging tools, the antenna may be tuned to specific frequencies. This tuning produces resonant frequencies with high Q for optimal performance. In general, the antenna is tuned using fixed value components such as resistors, capacitors and inductors. For protection, these components are usually mounted in a junction box or located under a protective sleeve near the receiving antenna. Because of the limited space available in a while-drilling tool, the junction box may be mounted in a cut-out of a steel structure of the tool.
Unfortunately, the conventional induction logging tool using a tuned antenna can have some drawbacks. One drawback is the need for the cut-out which increases the chances for corrosion and cracking in the tool structure. Another drawback is that the additional separate tuning circuitry adds more components to increase the chance of failure and, thus, increased time and cost required in a maintenance shop.
Therefore, what are needed are techniques to increase the reliability and performance of induction well logging tools.